Nuts & Volts

BY MICHAEL SIMPSON
consists of 12 desktop
PCs, three laptops, four
pocket PCs, and various
serial converters of
one type or another.
■ FIGURE 3
The Basic
Circuit
Take a look at
Figure 2. This is how
you would connect to a
desktop PC or laptop.
Pocket PCs are considered
DTE devices, so
you will need a male
connector and will
have to connect the
sensor output to pin 2
of the DB9 connector.
Don’t rush out
and build this circuit
because we are going
to do it one better.
I don’t want to
have to use a battery
or AC adapter, so I
decided to power the interface. By
adding a 78L05 and a diode, you can
create a self-powered version, as
shown in Figure 3.
We are using pin 4, DTR to power
the circuit. First, we run it through a
diode so we don’t blow anything up if
the DTR is set low. When low, the
cathode side of the diode will float,
effectively turning off the circuit. Once
DTR is raised, the positive voltage will
drive the 78L05 and get regulated
down to 5V. You don’t need much
since the whole circuit only pulls
a few milliamps.
Sensor Hookup
It already has a magnet mounted
on the main drive so I simply oriented
my sensor so that the front of the
sensor faced the magnet, as shown in
Figure 5.
I strapped the cable down with tie
wraps to help secure the sensor. If
your bike does not already have a
magnet, you will need to hot-glue one
to the drive mechanism somewhere.
Keep in mind that if you don’t want to
open up your bike, it is possible to
■ FIGURE 2
mount the sensor on the exterior of
the bike and a magnet on one of the
peddle arms. This is what I plan on
doing to one of my other bikes.
That’s it on the interface. It will
How you connect the sensor
depends upon your bike. All
bikes have some sort of rotating
part that the peddles are connected
to. The bike I have has an
onboard computer so it already
has a sensor. Figure 4 shows how
I just hot glued my sensor to the
top of the existing sensor.
■ FIGURE 4
■ FIGURE 5
March 2006 57